1. Field of the Invention
This invention relates to a pneumatic tire, and more particularly to a pneumatic radial tire of so-called run-flat type capable of running over a given distance at a state that an internal pressure is zero or near to zero due to puncture or the like and having particularly an excellent run-flat durability (running at a puncture state).
2. Description of Related Art
A run-flat type radial tire (hereinafter referred to as a run-10 flat tire) is mainly used in vehicles such as passenger cars or the like in which a load applied to the tire is relatively small, and is required to satisfy a condition that even if the tire is suddenly punctured during the running of the vehicle on a general-purpose road or on an expressway at a high speed, the vehicle, particularly the passenger car can safely be run without damaging the steering stability. Hence it can safely be run up to a location capable of exchanging the tire over a given distance, for example, 80-160 km without separating the tire out from the used rim (approved rim) even in the continuity of running and breaking the tire.
For this end, the run-flat tires having various structures are proposed, occasionally, in a combination with a fully worked-out special rim. However, the combination with the special rim is too expensive and is lacking in general-purpose use, but does not contribute to directly or indirectly improve the tire durability to an expected extent though there is recognized an effect of preventing the separation of the tire from the rim. Therefore, the run-flat tire itself will be described with reference to examples below.
In JP-A-55-68406 is proposed a pneumatic safety tire having an excellent running durability at puncture wherein a pair of thick rubber reinforcements having a crescent shape at its section are arranged in an inner face of a portion of an innermost carcass ply located over a zone ranging from a bead portion through a sidewall portion to an end of a tread portion or between portions of carcass plies. The rubber reinforcement has a JIS hardness of at least 70 degrees, a tensile stress M.sub.25 of not less than 10 kgf/cm.sup.2 after an aging test in an inert environment of 140.degree. C..+-.1.degree. C. for 24 hours, and a rebound resilience of not less than 65% through a Dunlop tripsometer.
In JP-A-1-278806 is proposed a safety tire wherein a sidewall reinforcing rubber layer having a crescent shape at its section is arranged in an inner face of a sidewall portion and divided into inner layer portion, middle layer portion and outer layer portion in a rotating axial direction of the tire. A soft rubber having a Shore A hardness of 50-70.degree. and a modulus at 100% elongation of 10-30 kgf/cm is applied to each of the inner and outer layer portions. A hard rubber having a Shore A hardness of 70-90.degree. and a modulus at 100% elongation of 30-70 kgf/cm is applied to the middle layer portion, and a bead apex rubber having a Shore A hardness of 74-95.degree. is further arranged.
In JP-A-3-176213 is proposed a run-flat pneumatic radial tire wherein a carcass has an up-down structure of 2 plies, and a reinforcing liner layer having a crescent shape at its section and made of rubber having a 100% modulus of not less than 60 kgf/cm and a 25 loss tangent at 100.degree. C. of not more than 0.35 is arranged inside the carcass ply in a sidewall portion, and a bead filler rubber having a JIS hardness of 60-80 degrees is further arranged.
In JP-A-4-345505 is proposed a pneumatic safety tire comprising carcass plies of up-down structure wherein a thick reinforcing rubber having a crescent shape at its section and divided into a first inner reinforcing rubber layer and a second outer reinforcing rubber layer in a radial direction of the tire is arranged in an inner face of an inner carcass in a sidewall portion and a bead filler rubber is further arranged. The Shore A hardnesses of these rubbers are made higher in order of the first reinforcing rubber layer, second reinforcing rubber layer and the bead filler rubber.
Among the aforementioned various proposals, the run-flat tire having an excellent cost performance and hence being put into practice in market is a pneumatic radial tire comprising radial carcass plies of up-down structure developing an effect of mitigating degree of collapse deformation during the running at the flat state as far as possible, a pair of thick reinforcing rubber of hard and soft strips disposed at an inner face side of an innermost turnup ply from a position near to a bead core in a bead portion through a sidewall portion to an end of a tread portion and having a crescent shape at its section, and a hard bead filler rubber enveloped between turnup ply and down ply and extending from an outer peripheral face of the bead core up to a position near to a maximum width of the tire. The term "up-down structure" used herein means a ply structure of 2 or more plies comprising a turnup ply wound around a bead core from inside of the tire toward outside thereof and a down ply enveloping the turnup ply form the outside. In such a tire, a rubberized layer of Kevlar cords or steel cords (called as an insert ply) may be arranged in a zone ranging from the bead portion to the sidewall portion.
This type of the tire is higher in cost as compared with general-purpose tires and is frequently used in any expensive vehicles such as sport car, sport-type car, high-grade passenger car and the like. Therefore, the run-flat tire mainly applies to a low-section profile tire having an aspect ratio of not more than 55.
Even in the tires having the above structures proposed in the aforementioned articles, however, if the internal pressure rapidly decreases to zero, the steering stability in the vehicle running at a high speed is sufficiently ensured, but it can be said that the high-speed running continuity at the run-flat state and the durability in the long-distance running are still insufficient. Therefore, it is desired to develop tires further improving the run-flat durability while ensuring a low cost possible as far as possible.
A main trouble form relating to the run-flat durability, which has been observed up to the present, is that as running at the flat state proceeds, a nucleus for separation failure is caused between the bead filler rubber and the turnup ply (inner ply) near to the bead portion. The separation nucleus then grows up to the position of maximum width in the sidewall portion, and hence a remarkably large crack is caused in the thick reinforcing strip rubber located slightly outward from the maximum width position toward the tread portion to finally cause the tire breakage not conducting running continuity